Ventilators are devices which are used to force gases, usually air or oxygen-enriched air, into the lungs of patients who, for one reason or another, are incapable of sustaining adequate ventilation entirely through their own efforts. The source of pressure may be a piston device, a built in blower or a high pressure line. Commercially-available ventilators utilize various mechanisms to regulate the pressure applied to the patient. In all cases, a breath is triggered which sets in motion a sequence of events during which pressure is applied until a volume or pressure target is reached, at which time the pressure cycle ends. Once the cycle is triggered, the ventilator proceeds in a predetermined manner, set by adjustments of dials on the control panel of the unit. With available devices and methods of ventilation, the ability of the patient to modulate breathing output through his own effort is limited or non-existent, as is apparent from the description below.
One of the principal indications for institution of ventilatory support is the presence of an unfavourable relation between patient effort and resulting ventilation (see FIG. 1). This may be because of neuromuscular weakness, necessitating a greater effort to produce a given pressure, abnormal respiratory mechanics with which greater pressure output is desired to attain a given ventilation, or both. This abnormal relation impairs the patient's ability to control ventilation and breathing pattern in a way that insures optimal CO.sub.2 removal and/or oxygenation. In addition, the high respiratory effort results in distress and, if the effort is sufficiently high, may ultimately lead to exhaustion (i.e. respiratory muscle fatigue).
By providing positive pressure at the airway during inspiration, all current approaches to ventilatory support (volume-, pressure- or time-cycled approaches) unload the respiratory muscles and improve the relation between patient effort and achieved ventilation in a global sense. For a given inspiratory effort, the patient receives a greater volume in the assisted breath than he would otherwise. The price of this support, however, is a variable degree of loss of control by the patient over his ventilation and breathing pattern. These effects are illustrated in FIGS. 2 to 4.
With volume cycled ventilation (FIG. 2), the ventilator delivers pressure in whatever amount and time pattern is necessary to cause a predetermined flow pattern and tidal volume to be achieved during the assisted breath. The operator of the ventilator (i.e. physician or therapist), and not the patient, determines the flow and volume to be delivered. If the patient makes an inspiratory effort during the inhalation phase, the ventilator simply decreases the pressure it provides in such a way that the flow and volume delivered are the same as prescribed. The more inspiratory effort the patient makes, the less the pressure delivered by the ventilator (FIG. 2, left to right). Conversely, if the patient does not wish to receive the prescribed volume or flow, and fights back, the machine generates a greater pressure to offset the patient's opposing effort. There results, therefore, an antagonistic relation between patient and machine.
With this volume cycled method of ventilation, the degree of loss of patient control over his own breathing varies, depending on the type of ventilation used. With continuous mandatory ventilation (CMV), loss of patient control is complete, as he not only cannot alter flow and volume within each assisted breath, but also cannot influence frequency. In the assist/control mode (A/C), the patient can alter the frequency of the mandatory breaths, but within each breath, the same adverse relation between patient effort and pressure delivered applies. With the newer synchronized intermittent mandatory ventilation (SIMV), spontaneous breaths are permitted between the mandatory breaths. During these breaths, the patient controls the rate and depth of breathing. However, the abnormal relation between effort and ventilatory output (see FIG. 1) which was the reason for installation of ventilatory support in the first place, continues to apply. In fact, it is worsened by the additional load imposed on the patient by the device and endotracheal tube. The patient, therefore, alternates between breaths in which effort is entirely without ventilatory consequence (mandatory breaths), and breaths in which effort produces abnormally low ventilatory return.
With pressure support methods of ventilatory assist (PS), the delivered pressure is a predetermined function of time, usually an intended square wave pressure beginning at the onset of inspiration and terminating as flow rate declines to a specific amount. The pressure delivered is, therefore, independent of how much inspiratory effort the patient is making during inspiration (see FIG. 3). Any inspiratory effort the patient makes during the breath would produce greater flow and volume (FIG. 3, A to C). However, since the pressure delivered is independent of effort, the ventilatory consequences of patient effort are still subject to the abnormal relation between effort and lung expansion dictated by the disease (FIG. 1). Although the overall relation between effort and ventilation is improved, the ability of the patient to alter ventilation in response to varying needs continues to be impaired. Furthermore, since patient effort normally increases in a ramp fashion during inspiration, while pressure delivered by the ventilator is nearly constant, the ventilator overassists early in inspiration while the assist decreases relatively as inspiration progresses. The patient then would sense an increase in load as inspiration is lengthened and this prompts the patient to breathe with short inspirations, resulting in a small tidal volume.
With airway pressure release ventilation (APRV), pressure at the airway alternates between high and low levels with a time sequence that is independent of patient effort (see FIG. 4). The periodic cycling of pressure insures a minimum ventilation. The patient can also obtain spontaneous breaths independent of the programmed cycles. During these, he receives no assist (i.e., similar to SIMV). The relation between effort and ventilatory consequences during these breaths continues to be poor, as dictated by disease, limiting the patient's ability to alter flow and ventilation in response to varying needs. In fact, since with APRV, operating lung volume is increased, the relation between effort and ventilatory consequences is further compromised on account of the well established adverse effect of increased lung volume on neuromechanical coupling (i.e. pressure generation for a given muscle activation).
I am aware of specific prior art proposals to effect modifications to commercially-available pressure-powered ventilators to allow the pressure produced to vary with electrical activity recorded from a respiratory nerve, as described in Remmers et al, "Servo Respirator Constructed from a Positive-Pressure Ventilator", J. Appl. Physiol. 41: 252 to 255, 1976. To the extent that activity in inspiratory nerves reflects effort these modifications would permit a ventilator to deliver pressure in proportion to effort, as is intended in the present invention. These modifications, which were developed for animal use where inspiratory nerves are accessible and can be recorded from, implicity require direct measurement of inspiratory muscle or nerve activity which is not practical in humans requiring ventilatory support. In contrast, the present invention permits the delivery of pressure in proportion to patient effort without the need for direct recording of activity, and through the use of algorithms that permit the inference of degree of effort from easily measurable variables, such as flow and volume. Poon et al, "A Device to Provide Respiratory Mechanical Unloading", IEEE Trans. Biomed. Eng. 33: 361 to 365, 1986, described a modification to a commercially-available volume ventilator which permits the ventilator to deliver pressure in proportion to inspired flow. Although this device was developed originally to simulate and amplify the effect of helium in reducing respiratory resistance during experimental studies on exercising humans, it can theoretically be used in patients to provide partial ventilatory support. As such, the device would develop pressure with a time pattern that resembles inspiratory flow, which is highest early in inspiration and declines later. Since this pattern is poorly (or even negatively) correlated with patient inspiratory effort, which rises continuously during the inspiratory phase, this pattern of support is quite unlike the method of the present invention, where pressure is intended to be a function of inspiratory effort throughout inspiration, as described in more detail below. Not only is the method completely different (simple resistive unloading vs assist in proportion to effort) but the apparatus described herein is much more suitable for our method (Proportional Assist Ventilation, PAV) than a modification of existing volume ventilators which are designed to regulate flow and not pressure. The design of the invention permits unlimited flow and there is no delay between onset of inspiratory effort and onset of flow from ventilator to patient, since no triggering is required before the gas delivery system is communicated to the patient. The system of the invention also can effect proportional assist through positive pressure at the airway or negative pressure at body surface, whereas a modified positive pressure, gas powered ventilator can serve the former function only.
Some known prior art patents describe a variety of breathing devices. U.S. Pat. No. 3,985,124 describes a spirometer for measurement of the rate and volume of expiratory breathing to create a graphic record of the same. This device possesses an expansible chamber of the piston type which expands in proportion to the exhaled air.
U.S. Pat. No. 3,669,097 describes a device for increasing the capacity and strength of lungs. An expansible bellows chamber is connected to a conduit having a mouthpiece. A selectively-adjustable valve is present in the conduit for constricting the passage from the mouthpiece to the inlet to the bellows chamber, so that a force in excess of the normal pressure developed by the lungs is required to expand the bellows.
U.S. Pat. No. 4,301,810 describes a ventilatory muscle training apparatus comprising a reservoir and a mouthpiece and also having a simple valving system to vent stale air from the reservoir during exhalation and let fresh air into the reservoir during inhalation. The air flow through the mouthpiece is monitored to ensure the intended manner of use of the apparatus is maintained.
U.S. Pat. No. 4,462,410 describes a recording spirometer for performing a breath test which uses a movable pusher plate which is moved in response to the breathing of the patient and a recording medium which enables a record to be made of the volume of air expelled by a patient as a function of time.
U.S. Pat. No. 4,487,207 describes a lung exercise device which has a mouthpiece through which a patient inhales. A conduit connects the conduit to an air inlet and a valve is located in the conduit, normally biased to a closed position. Upon inhaling, the valve is opened and the amount of air inhaled is monitored.
U.S. Pat. No. 4,459,982 by Fry discloses a lung ventilator device which comprises a chamber means which delivers respiratory gases to a patient. This patent describes, as one embodiment, the provision of a flow rate directly controlled by the patient's instantaneous demand under spontaneous breathing of the patient, and hence, at first sight, may be considered relevant to the present invention. However, the operation of the device and its control require movement of the piston in the chamber to maintain the pressure at the patient's airway constant and equal to a reference pressure determined by the operator. Since the device operates to maintain airway pressure constant throughout inspiration, it is apparent that it does not deliver pressure in proportion to patient effort (which varies throughout inspiration), as is the case of the present method (PAV).
British Patent No. 1,541,852 describes a piston, driven by a motor which alters the pressure in the piston according to the power supplied to motor. This system is designed to deliver pressure according to a predetermined pressure-time profile (as in pressure cycled ventilators) or to force a given volume of gas into the patient (as in volume cycled methods). In the method of the present invention, neither pressure nor flow and volume is/are predetermined. Rather, the patient determines his own flow pattern and tidal volume through his own effort, while the ventilator delivers pressure in a manner that parallels the patient's ongoing effort (which is obviously not predetermined).
Other representative prior art employing motor driven pistons that deliver predetermined pressure vs time or volume vs time patterns know to the applicant are Hillman, U.S. Pat. No. 4,036,221, Chu, U.S. Pat. No. 4,617,637 and Apple, U.S. Pat. No. 4,726,366.
Stawitcke (U.S. Pat. No. 4,448,192, U.K. Patent No. 2,121,292) describes a system with motor driven pistons and extremely complex controls, the intent of which are to reduce conflict between patient and ventilator. A control system continuously computes an ideal pressure-volume trajectory designed to cause the ventilator to deliver gas in the amount (tidal volume) and flow rate specified by the physician while allowing for different degrees of patient effort. Although this system permits the patient to over-ride physician-specified delivery pattern within an inhalation or over a brief period, the control system readjusts the terms in the equation, so that physician-determined criteria are ultimately met. The principle of control is, therefore, similar to volume-cycled methods, in that an increase in patient effort is met with a decrease in machine assist to return ventilator output to what the physician prescribes. The main difference from other volume-cycled methods is in the freedom the patient is accorded to transiently over-ride the prescriptions by the physician. This principle of operation is diametrically opposite to that executed by the method of the present invention. Thus, in the Stawitcke system, the physician sets targets for volume, flow and timing and the machine alters the various parameters in the control system equation in such a way as to meet physician requirements. By contrast, in the present invention, the proportionalities between pressure, on one hand, and volume and flow on the other hand are the parameters that are predetermined, while the patient is left entirely free to select volume, amount and pattern of flow, and timing of each breath.
It will be apparent from this discussion, that prior art patient ventilation systems provide ventilatory support to a patient in accordance with parameters which are determined mainly by a physician and not by the patient. Generally, the prior art has required some target flow rate, target pressure, target volume, and/or target frequency or inspiratory or expiratory time. Such requirements give rise to the various problems discussed above.